Various attempts have been made to increase photosensitivity of image sensors. One proposal is a photo-gathering technology. For instance, a CMOS image sensor may include a photodiode to detect light and a CMOS logic circuit to convert detected light into electric signals representing data. To improve the photosensitivity, a fill factor, which may be a ratio of a photodiode area to the whole area of the image sensor, could be increased.
Because, however, the CMOS logic circuit is an essential element of an image sensor, a problem may occur if the CMOS logic circuit is removed from the image sensor. Thus, there may be limitations on improvements to the photosensitivity of the image sensor based on the limited area of the image sensor.